1. Field of the Invention
The present invention relates to a data transmission system utilizing a power line. More specifically, the present invention relates to a data transmission system utilizing a power line for transmission of data in a bit serial code fashion in synchronism with the cycles of alternating current of a power supply.
2. Description of the Prior Art
A system for transmission of data utilizing a power line has been conventionally proposed and put into practical use. For example, a system for controlling various indoor electric appliances by superposing a high frequency carrier as control data on an alternating current wave of a power supply by the use of an indoor supply line has been put into practical use. In general, the data high frequency modulated in synchronism with the cycle of the alternating current of a power supply is superposed on an alternating current signal. The prior art of interest in transmitting necessary data through superposition of a signal high frequency modulated on such power supply alternating current wave is disclosed in Japanese Publication Gazette No. 34046/1970 filed July 7, 1966 by Matsushita Electric Industries Ltd. and published for opposition Nov. 2, 1970. The above referenced Japanese Patent Publication Gazette teaches that the phase position, i.e. the phase angle, of the alternating current is used to represent particular data information. More specifically, a half cycle of the power supply cycle, i.e. a time period from the phase of the voltage being zero to the subsequent phase of the voltage becoming zero again after a half cycle is divided into a plurality of phase sections, say 6 sections. Each of these sections, as divided, is allotted to represent in succession Channel Nos. 1, 2, . . . 6. Therefore, if and when a high frequency is superposed only on the second section of the half cycle, then the phase position of the second section is treated as the data for identifying Channel No. 2. Another improved system of interest is disclosed in Japanese Patent Publication Gazette No. 34047/1970 filed Aug. 17, 1966 by Matsushita Electric Industries Co., Ltd. and published for opposition Nov. 2, 1970. More specifically, the referenced patent publication gazette teaches that the respective periods of the positive and negative polarities of a power supply alternating current are discriminated and the positive and negative polarity periods are combined to form two regions, whereby the number of channels two times that of the first referenced Japanese Patent Publication Gazette No. 34046/1970 is available. However, these systems disclosed in the above described two referenced Japanese Patent Publication Gazettes are adapted to represent different data for each of the divided sections by dividing a half cycle or one cycle of the alternating current into a plurality of sections and for this reason the number of channels is restricted to the number of division. Accordingly a difficulty is caused in increasing the number of division and hence a disadvantage is caused that the quantity of information being transmitted is limited. Furthermore, since the data available in one channel is only the logic one or zero, the quantity of data available in one channel is accordingly limited. In addition, since the data is determined in terms of the phase position or the phase angle, it is extremely difficult to achieve accuracy. On the other hand, an advantage is brought about by the above described prior art in that because of complete time division no problem of conflict is caused on the occasion of simultaneous operation of a plurality of channels.
A further system of interest to the present invention publicly available is a wireless remote control unit manufactured and sold under the name of system X-10 by BSR of the United States. The system X-10 utilizes power line transmission technology. With the system X-10, a clock is generated for each half cycle of the power supply alternating current. Then a bit serial code is provided using a plurality of cycles by using each half cycle of the alternating current as a reference, i.e. by allotting one bit to each half cycle. However, the X-10 system merely employs each half cycle as one bit and the system has not employed any countermeasure for an interference due to a noise or the like. Accordingly, a disadvantage is involved with the X-10 system in that the information is changed by one bit due to noise occurring during a half cycle or attenuation of a high frequency signal as superposed, resulting in malfunction.
Still a further system of interest to the present invention is disclosed in Japanese Utility Model Laid Open Gazette No. 59130/1979 filed Sept. 30, 1977 by the same assignee as the present invention and laid open for public inspection Apr. 24, 1979 and in Japanese Patent Laid Open Gazette No. 50940/1979 filed Sept. 30, 1977 by the same assignee and laid open for public inspection Apr. 21, 1979. More specifically, these laid open gazettes show that a half cycle of the alternating current is basically utilized to allot the address data of one channel in each half cycle, as done in the previously described prior art. However, according to the last described Japanese Laid Open Gazettes, each half cycle is divided into a plurality of sections and the channel data is coded depending on the divided bit positions in terms of the phase positions or angles. In transmitting the data, a start pulse is allotted in one half cycle and the channel order or sequence information is allotted in succession in the respective half cycles following the start pulse. The order or sequence information is represented by a bit serial code distributed over the half cycle. However, according to the prior art now in discussion, since the channel position is determined by the half cycle, a problem is caused that, when the number of channels is increased, designation of those channels being designated later in the order requires a prolonged response time period. Accordingly, the number of channels is restricted. Furthermore, no countermeasure exists for eliminating a conflict occurring in the case where the transmitted data changes due to a noise other than the data information coming in the alternating current waveform or in the case where attenuation of a signal high frequency modulated through superposition on the alternating current waveform occurs.